Recently along with increasing concern about traffic safety, various airbags have been developed for assured safety of passengers on car accident and commercialized rapidly because of recognition of the effectiveness thereof.
An airbag protects a driver or a passenger by expanding itself in a quite short period of time after vehicle collision and thus, capturing the passenger thrown away in reaction of the collision and absorbing the impact then. For that purpose, the fabric for the bag desirably has low air permeability. The fabric should also have a strength higher than a particular value for endurance to the impact during inflation of the airbag. It is further needed to reduce the yarn displacement, i.e., to improve yarn displacement resistance, in the sewn region as much as possible, to keep the internal pressure in the bag at a particular value or higher for inflation of the airbag and protection of passengers. In addition, compactness in packing is desirable from the points of appearance in the car and relationship with other parts, and there is increasing demand for cost reduction.
Coated fabrics, i.e., airbag fabrics coated with a resin or bonded to a film, have been proposed as means for reducing the air permeability of fabric.
However, resin coating or film bonding is unfavorable for airbag fabrics, because it leads to increase in fabric thickness and decrease in compactness in packing. In addition, addition of such a resin coating step or a film-bonding step also causes a problem of increase in production cost.
To solve the problems above, recently proposed were uncoated fabrics having low air permeability that were prepared by weaving synthetic filament yarns such as of polyamide or polyester fiber at high density without resin processing. For example, disclosed as a means for improving air permeability is a method of using a fabric in a symmetrical fabric structure that was prepared by using a synthetic filament yarns having a yarn fineness of 300 to 400 dtex (see, for example, Patent Document 1). If prepared by the method, a fabric consisting of warp and weft yarns of synthetic fiber yarn having a fiber fineness of 300 to 400 dtex at a yarn counts of 23 to 28 yarns/cm in a symmetric fabric structure, wherein the warp yarns and the weft yarns are woven substantially symmetrically in yarn density, has an air permeability of 10 L/dm2·min or less at a test pressure difference ΔP of 500 Pa and shows mechanical properties isotropic in the warp and weft yarn directions.
However, the method demands production of a fabric containing the warp and weft yarns respectively at a density of 23 to 28 yarns/cm, for obtaining favorable air permeability and particular mechanical performances. For that reason, if the edgecomb resistance, an indicator of the degree of airbag yarn displacement in the sewn region by inflation of the airbag for passenger protection, is examined, there is a problem of ill-balance thereof in the fabric warp and weft directions.
On the other hand, known for improvement in productivity while making the base cloth isotropic in strength and flexibility, is a method of specifying the so-called density coefficient rate defined by Formula: {weft yarn density (yarns/cm)×(weft yarn fiber fineness (denier))1/2}÷{warp yarn density (yarns/cm)×(warp yarn fiber fineness (denier))1/2}, (see, for example, Patent Document 2).
The method allows reduction the difference in bending resistance between in the fabric warp and weft directions if the density coefficient rate is adjusted to 0.92 or less, and thus, allows improvement in airbag compactness in packing.
However, the document only focuses on isotropy in bending resistance. Reduction of air permeability, the most important property for airbag, was not even mentioned there. In addition, when the edgecomb resistance is examined, the fabric edgecomb resistance in the weft direction is extremely low, compared to that in the warp direction, causing a problem of extremely ill-balance between the edgecomb resistances in the warp and weft directions.
In addition, disclosed is an airbag fabric wherein the weaving density of warp yarns and the weaving density of weft yarns are made different from each other for improvement in productivity while keeping the airbag isotropic during inflation (see, for example, Patent Document 3).
However, the document discloses fabrics wherein the weaving density of warp yarns is higher than the weaving density of weft yarns. In the fabrics prepared by the method, the fabric edgecomb resistance in the weft direction is extremely lower than that in warp direction, causing a problem that the balance in the edgecomb resistances in the warp and weft directions is very low.
Alternatively, an airbag base cloth woven at ultrahigh weaving density was disclosed as a base cloth superior in the yarn displacement resistance in the sewn region (see, for example, Patent Document 4).
However, the invention described in the document uses a fabric having high weaving density as the means of raising the edgecomb resistance. Thus, the fabric is inferior in the compactness in packing needed for airbag, and thus, is not a base cloth superior both in edgecomb resistance and compactness in packing.
As described above, there is no airbag fabric produced by conventional technology that is superior all in low air permeability, high strength and compactness in packing that are needed for airbag fabrics and smaller in yarn displacement in the airbag sewn region when the airbag is inflated for protection of passenger.
Patent Document 1: Japanese Unexamined Patent Publication No. 3-137245 (Claim 1)
Patent Document 2: Japanese Unexamined Patent Publication No. 2001-200447 (Claim 1, Paragraph 0013)
Patent Document 3: Japanese Unexamined Patent Publication No. 2000-303303 (Claims 3 and 7, Paragraph 0038)
Patent Document 4: Japanese Unexamined Patent Publication No. 2006-16707 (Claim 1)